A mouse monoclonal antibody (MAb, 4B6) was able to recognize dengue virus type 4 envelope (E) protein both as a recombinant protein in Pichia pastoris and when it was present in infected brains of suckling mice. 4B6 was characterized by enzyme-linked immunoadsorbent assay (ELISA), hemaglutination inhibition, neutralization, and immunoblot. The MAb was isotyped as IgG2a. It was serotype 4 specific and it inhibited hemaglutination and neutralized homologous virus. It did not enhance infection of P338D1 cells by dengue type 4 virus strain H-241 strain. This MAb was reactive with recombinant E protein and dengue 4 virus, as revealed by Western blot. In vivo, MAb 4B6 conferred passive protection in mice challenged with homologous virus. Currently, this MAb is being used to purify recombinant E protein for further studies.